Cell culture genetic experiments with the Rana pipiens embryo haploid line ICR 2A (ATCC/CCL 145) and its derivatives are intended to explore the relative importance of mutational and epigenetic factors in the origin of variant clones in vitro. The origin of resistance to 5- bromodeoxyuridine (BUdR) is being examined by characterizing thymidine phosphorylating enzymes from wild-type and BUdR-resistant cells of 3 phenotypic classes. Variations in the competence of thymidine transport-deficient subclones to yield thymidine kinase-deficient clones are being related to effects of cell density, mutagen treatment and other culture conditions. Haploid-diploid comparisons of lethality after X-rays and chemical mutagens are being made by both single cell survival and growth curve methods. The latter is used to discriminate among several phases of cell growth and lethality that follow mutagen treatment. We will evaluate a potential method for mutagen testing based on the difference in expected lethality between haploid and diploid clones in which unrepaired recessive lethals have been induced. Nuclei of drug-resistant haploid cells will be transplanted to enucleated eggs to determine if these phenotypes can be transmitted as nuclear traits.